Recently, RFID (Radio Frequency Identification) tags (wireless tags) have been widely used. The RFID tags are used as a substitute for barcodes. The RFID tags are gathering popularity particularly in the field of distribution and are expected to spread widely in the near future.
Presently, as a frequency band exclusively for the RFID tags purpose, 13.56 MHz band, a so-called UHF band that is somewhere between 800 MHz and 950 MHz, and 2.45 GHz band are used. Among these bands, radio waves using the UHF band or the 2.45 GHz band are advantageous over radio waves using the 13.56 MHz in that it is easy to increase a communication range. In addition, the UHF band radio waves are advantageous over the 2.4 GHz band radio waves in that it is easy to go behind the object. For this reason, RFID tags and reader/writers using the UHF band radio waves are being developed.
In the case of using the UHF band radio waves, it is possible to increase the communicable distance between the reader/writer and a wireless tag from several ten centimeters to several meters, compared to the case of using the 13.55 MHz band radio waves which are widely used at present. Accordingly, by utilizing the UHF band radio waves, it becomes possible to relatively greatly increase a communication area which is a spatial area in which the reader/writer can communicate with the wireless tags.
Meanwhile, in order to estimate the location of the RFID tags, there has been suggested a technology for measuring the distance between the RFID tags and a communication station in communication with the RFID tags. As an example, a technology is known in which a plurality of base stations receive signals from an active IC tag as the RFID tag, and the distance to and location of the active IC tag are estimated on the basis of the reception signals from the active IC tag. In the example, the distance between each of the base stations and the active IC tag is estimated on the basis of the signal intensity of the reception signals from the active IC tag. That is, the distance estimation is performed by utilizing correlation between the reception signal intensity and the distance. Also is known a method in which by preparing access points of which the location is known, signals are received simultaneously from the active IC tag and the access points, and the delay amount between receiving timings for the respective signals, thereby estimating the distance to the active IC tag.
As shown in FIG. 26, Japanese Patent Publication No. 2004-507714T (published on Mar. 11, 2004; hereinafter will be referred to as Patent Document 1) discloses an RF communication system in which an interrogator 36 as a reader transmits signals 40 and 42 having different frequencies to an RF tag 38, and the number of null points in combined waves that are obtained by superimposing the two signals onto each other is counted, thereby estimating the distance between the interrogator 36 and the RF tag 38.